Bokeh is a term that's become increasingly common in recent years. It’s a Japanese word that simply refers to the aesthetic quality of out of focus parts of an image, which are usually in front and behind a sharp and in-focus subject. It’s achieved by shooting with a wide-aperture lens, such as one with a maximum aperture of f/1.8, or, in the case of longer telephoto lenses, f/2.8.
Lens design plays an important role in the appearance of bokeh, with lenses that have more blades in the aperture diaphragm said to produce more pleasing bokeh. This is because when set to their maximum aperture they produce a more circular opening; lenses with fewer diaphragm blades produce a more polygonal shape, which becomes evident when there are distinct highlights in the background, and which is considered less pleasing to the eye.
While bokeh most commonly refers to the out-of-focus areas in front of and behind a subject, it can also be used as an abstract technique in itself. By shooting with a lens at its maximum aperture and throwing highlights out of focus, they can become soft and hazy discs – this approach is commonly used with fairy lights as the main subject.
A variation that has also become popular is to use fairy lights in portraits where the lights lead out to the camera from the model, so they’re captured as soft round highlights that become more defined the closer they are to the plane of focus.
If you only have a kit lens, such as an 18-55mm, the best way to maximise its bokeh potential and increase background blur is to zoom it to its longest focal length of 55mm. Then set the maximum (widest) aperture of f/5.6. You'll then need to move into position to compose the shot, rather than zooming.
However, this will never produce the same quality of bokeh as a 50mm f/1.8 lens, for instance, so if you want to dramatically improve the quality of the bokeh in your images then a 'fast' 50mm lens, or another one with a large maximum aperture, is well worth considering.